A valve actuator is designed to open/close a valve at the time of emergency. For example, when power is shutoff due to the fire etc., the valve actuator opens the valve, whereby air vent at the facility is opened. Also, when an accident happens in a ship or chemical industry facility, the valve should be closed automatically in order to avoid a collision or hazardous gas.
There are two types of valve actuator. One is pneumatic type actuator operated by compressed air; the other is motor driving type actuator operated by electricity. In case of pneumatic type actuator, the preparation of compressed air is indispensable. Therefore, motor driving type actuator is easier to use and more usual type. The motor driving type valve actuator is driven by emergency power or by the driving force of an elastic member to shutoff a valve at the time of emergency. Generally, using the driving force of an elastic member, e.g. a spring, especially a coil spring, on cite is more common than using the emergency power driving.
A conventional spring return valve actuator has very simple structure, in case of small type, comprising a motor, a worm member connected thereto, and a planetary gear member. A valve is connected to a sun gear shaft (final output shaft) of the planetary gear, and an elastic member is fitted between the sun gear shaft and the valve. On the other hand, more complicated structure is applied to a large type spring return valve actuator. The large type actuator comprises a first worm member, a second worm member and two planetary gear connected to the second worm member, and an elastic member is fitted onto the final output shaft. In this spring return valve actuator, the driving force of the motor is transmitted from the first worm member to the second worm member, and to the first planetary member, then the second planetary member in this order, whereby the valve arranged on the second planetary member is opened/closed normally. On the other hand, at the time of emergency, e.g. power shutoff, a brake which connect to the first worm member is released, and the valve is opened/closed by the force of the elastic member which is fitted onto the final output shaft (second sun gear shaft).
However, the conventional spring return valve actuator has some defects such that an elastic member having large elastic coefficient and tensile force should be used in order to drive the valve directly since both of the elastic member and the valve are fitted onto the final output shaft together, that the lifetime of the valve actuator is not as long since a large load will be applied on the output shaft, and that the volume of the valve actuator is large.